This invention relates to xerographic reproducing apparatus and, in particular, a control device for such an apparatus wherein a drive system for the photosensitive drum and the roll fuser are controlled at a high speed when it is desired to produce a small number of copies from, for example the same original, and at a combination of high and low speeds when a large number of copies are produced.
In a prior art copier having a drive system as shown in FIG. 1, a photosensitive member b and a fixing roll are driven by a drive motor a, so that a toner image formed on the surface of the photosensitive member b is transferred to sheet e fed from a sheet tray d, and the toner image is fixed to the sheet e by the fixing roll c. In a copier of this type, the copying speed has been set to the speed of a system which is the lowest in the follow-up characteristic, in order to adjust a balance in speed among an optical system, a development system and the fixing system. This, however, makes it difficult to increase a copying speed in such a copier, coupled with the limitation on an available power source plug socket. Particularly in a small sized copier, the copying speed is largely dependent on the fixing temperature, which has been an obstacle to increasing the copying speed of the small size copier. For example, when copies are produced under high humidity conditions using a fixing device having a temperature characteristic as shown in FIG. 2 (A--A' represents a duration of reproduction), the fixing temperature is lowered to T.sub.2 shown in FIG. 3 by the time a large number of copies have been produced, resulting in the incomplete fixing of a toner image due to temperature fall-off of the heated fuser roll to an inadequate fusing temperature. Taking into consideration the fact that, in most cases, less than ten copies are produced from the same original, if a drive system is so arranged as to be controlled at a high speed when a small number of copies are produced per unit time, and at a high speed followed by a low speed when a large number of copies are desired, even a small size copier resorting to a power source plug socket in general use may be increased in copying speed.
It is accordingly intended to provide a control device for a copier, wherein a drive system is controlled at a high speed where a small number of copies are produced from a same original and at a high speed followed by a low speed where a large number of copies are desired, whereby increase in a copying speed of a copier employing a power source plug socket in general use is attained.
It is known in the prior art to rotate fuser rolls at different speeds, for example, as described in U.S. Pat. No. 4,081,213. In this patent the speed of the fuser rolls is controlled by the "on" and "Print" buttons of the machine and not by a signal generated by sensing the fuser roll temperature. Moreover, the purpose of the roll rotating at a slower speed is but one of the means by which a soft fuser roll is prevented from being deformed when copies are not being fused. Accordingly, there is only one process speed for the fuser and that is the higher of the two speeds.